1. Field of the Invention
The present invention relates to an optical information recording and/or reproducing system and, especially, to an optical information recording and/or reproducing system for recording or reproducing information in parallel on or from a recording medium such as an optical disk by a plurality of laser light sources.
2. Related Background Art
In recent years, research and development for improving a transfer rate for transferring data have been energetically conducted with respect to an optical information recording/reproducing apparatus, such as a magneto-optic disk apparatus. In the magneto-optic disk apparatus offered as a product at the present, the disk must make three revolutions for erasing/recording/reproducing (verifying) processes when writing data. Such magneto-optic disk apparatus have a substantially lower data transfer rate than that of an apparatus using a hard disk.
Under such circumstances, there have been proposed an overwritable information recording medium and an optical information recording/reproducing apparatus capable of overwriting, wherein recording and erasing processes are performed for a duration of one revolution of the disk. An optical information recording/reproducing apparatus for verifying information immediately after recording by use of a plurality of beam spots and an optical information recording/reproducing apparatus for recording and/or reproducing the information in parallel by employing the plurality of beam spots has also been proposed. For instance, a system for modulating a magnetic field applied to a magneto-optic recording medium in accordance with the information recorded on the recording medium has been proposed in the form of a magneto-optic disk apparatus capable of overwriting. Also proposed in addition to the above system was a magneto-optic disk apparatus capable of performing all the erasing/recording/reproducing processes for a duration of one revolution of the disk, thereby enhancing the data processing speed of the magneto-optic disk. This is accomplished by irradiating a track on the disk with a plurality of light beams for use of recording and verifying as well to form beam spots, respectively. Such systems are all based on a magnetic field modulation overwrite system for recording the information by modulating the magnetic field applied to the magneto-optic recording medium. On the other hand, there has also been proposed a magneto-optic recording medium which is overwritable by modulating the light beams for writing the information. This recording medium has a structure in which multi-layered magnetic films having different Curie temperatures and coercive forces are exchange-coupled. The recording medium is employed in the magneto-optic recording/reproducing apparatus of the optical modulation overwrite system. Further, there has been proposed an apparatus for recording and reproducing information in parallel by forming a plurality of beam spots on track turns adjacent to each other on the magneto-optic recording medium. This apparatus records and reproduces in parallel by use of substantially the same magneto-optic disk apparatus as the conventional one employing a single light source, by using a semiconductor laser array as a light source. In this manner, the magneto-optic disk apparatus is capable of enhancing the data transfer rate up to the level of a hard disk or higher by exploiting the capability of multiplexing the plurality of light beams.
There is known e.g., a technique of recording and reproducing the information by forming beam spots in parallel respectively on a plurality of track turns adjacent to each other in the radial direction of a recording medium when irradiating a magneto-optic recording medium with beams emitted from a semiconductor laser array. In this case, the recording and reproduction of the information at a high transfer rate can be easily attained by using the magneto-optic recording medium having a helical track. The following problems, however, arise when the magneto-optic disk apparatus capable of recording and reproducing the information in parallel by use of the plurality of beam spots records and reproduces the information in parallel on and from the magneto-optic recording medium, presently available as a product having a single helical track. This will be explained with reference to FIGS. 1 and 2. FIG. 1 schematically illustrates the magneto-optic recording medium having a helical track. In FIG. 1, the numeral 1 represents a center of revolution; 2 designates a single helical track; and an arrowhead indicates a direction of revolution of the recording medium. Beam spots 3-1, 4-1 of the magneto-optic disk apparatus for recording and reproducing the information in parallel as described above are positioned on track turns adjacent to each other. For instance, it is assumed that the beam spot 3-1 starts recording the information from an arbitrary point A on the magneto-optic recording medium. It is similarly assumed that the beam spot 4-1 starts recording the information from a point B. The points A, B are, as is obvious from the Figure, located in close proximity on track turns adjacent to each other. In this state, when the magneto-optic recording medium makes one revolution, that is, explaining the revolution of this magneto-optic recording medium with the aid of movements of the beam spots, the two beam spots come on the downstream side for one track upon recording the information for the one track. More specifically, the beam spot 3-1 reaches the point B, while the beam spot 4-1 comes to the point C. The beams spots moved are expressed by numerals 3-2, 4-2, respectively.
If the recording information exists over two or more tracks, however, as illustrated in FIG. 2, for example, the beam spot 3-2 is required not to overwrite the information already recorded by the beam spot 4-1 from the point B. For this purpose, the two beam spots have to jump tracks to adjacent track turns. It is, however, impossible to instantaneously move to the adjacent tracks. Hence, a recordable state is not present until a track jumping time or a tracking servo settling time after jumping has taken place. This will be explained referring to FIG. 2. The beam spot 3-2 is incapable of recording until the beam spot 3-2 jumps the track from the point B and reaches a point c. The beam spot 4-2 is similarly incapable of recording until the beam spot 4-2 jumps the track from the point C and reaches a point d. Namely, at least a portion (C-c) corresponding to a section L is not effectively used. When the information is continuously recorded on the magneto-optic recording medium, and if continuously recorded on, e.g., an A-E section, the C-c section is blank, or a completely different item of information already recorded on the medium may be left in case the magneto-optic recording medium is overwritable. Then, when making an attempt to reproduce the above recording information by the magneto-optic disk apparatus serving as a recording/reproducing means using the single beam spot which is now being manufactured into reproducing apparatuses, it can be easily imagined that some drawbacks appear therein. That is, it is difficult to make compatible a magneto-optic disk apparatus capable of recording and reproducing the information in parallel and a magneto-optic disk apparatus using a single beam spot. This is quite inconvenient. Further, a wait for revolution without jumping the track may be possible for avoiding the above-stated situation until the beam spot 3-2 reaches the point C, and the beam spot 4-2 comes to the point D. If this is done, however, the enhanced transfer rate made possible by simultaneously recording and/or reproducing the information with the plurality of beam spots cannot be taken advantage of.